This invention relates to a sheet feeding mechanism having a tray for accommodating a stack of sheets used in an apparatus such as an image forming apparatus, and relates to an image forming apparatus having the tray.
FIG. 1 is a perspective view of a conventional sheet feeding mechanism used in an image forming apparatus. FIG. 2 is a schematic view of the conventional sheet feeding mechanism. As shown in FIG. 1, the sheet feeding mechanism includes a tray 101 in which a stack of sheets is accommodated. The tray 101 is inserted in a not-shown main body of the image forming apparatus. The tray 101 has a sheet feeding side 101a (i.e., the lower-right side of the tray 101 in FIG. 1) through which the sheets are fed out of the tray 101.
A pickup roller 111 and feeding rollers 112 and 113 are rotatably provided in the main body, and are located at the sheet feeding side 101a of the tray 101 inserted in the main body. The pickup roller 111 and the feeding rollers 112 and 113 are respectively rotated by motors (not shown) provided in the main body.
The tray 101 has a swingable base plate 102 supported by a support shaft 102a (FIG. 2). The tray 101 has adjustable guide plates 103, 104 and 105 guiding three sides of the stack of the sheets placed on the base plate 102. The guide plate 103 guides a side of the stack opposite to the sheet feeding side 101a. The guide plate 104 and 105 guide both sides of the stack in the direction perpendicular to the sheet feeding direction, i.e., the direction in which the sheets are fed out of the tray 1.
For swinging the base plate 102, a shaft 106 is rotatably provided in a case 108 attached to the sheet feeding side 101a of the tray 101. The shaft 106 has an end 106a projecting out of the case 108, and pins 106b are formed on the end 106a. The end 106a of the shaft 106 engages a coupling 110 of a motor 109 provided in the main body. As shown in FIG. 2, an L-shaped lifting member 107 is fixed to the shaft 106. When the shaft 106 rotates clockwise in FIG. 2, the lifting member 107 swings upward and abuts against the bottom of the base plate 102, so that the lifting member 107 lifts the base plate 102 toward the pickup roller 111. As a result, the uppermost sheet of the stack on the base plate 102 abuts against the pickup roller 111.
As shown in FIG. 1, for determining the position of the tray 101 in the main body, a holding member 115 is movably provided in the main body. The holding member 115 fits in a hole 114 formed on the case 108 of the tray 101, so that the tray 101 is held at a predetermined position in the main body. The holding member 115 is urged by a spring 116 in a direction in which the holding member 115 is inserted in the hole 114.
When a user inserts the tray 101 into the main body in the direction indicated by an arrow A (hereinafter, referred to as the direction A), the end 106a of the shaft 106 engages the coupling 110, and the holding member 115 fits in the hole 114, with the result that the tray 101 is fully inserted in the main body. Then, as shown in FIG. 2, the motor 109 rotates clockwise and the lifting member 107 swings upward to lift the base plate 102 toward the pickup roller 111. The height of the base plate 102 is detected by a detection unit 117. The rotation of the motor 109 is stopped in such a manner that the uppermost sheet of the stack on the base plate 102 abuts against the pickup roller 111. The positions of the lifting member 107 and the base plate 102 are respectively indicated by numerals 107U and 102U in FIG. 2. As the pickup roller 111 and the feeding rollers 112 and 113 rotate, the uppermost sheet is fed out of the tray 101.
As shown in FIG. 1, when the user pulls the tray 101 in a direction opposite to the direction A, the end 106a of the shaft 106 disengages from the coupling 110 of the motor 109. As the power of the motor 109 is not transmitted to the shaft 106, the lifting member 107 and the base plate 102 swing downward because of their weight, so that the uppermost sheet of the stack on the base plate 102 moves away from the pickup roller 111. The positions of the lifting member 107 and the base plate 102 are respectively indicated by numerals 107L and 102L in FIG. 2. In this state, the user pulls the tray 101 out of the main body.
However, in the above-described conventional sheet feeding mechanism, the lifting member 107 and the shaft 106 are provided in the tray 101, and therefore the weight of the tray 101 increases. Because of the weight of the tray 101, it is not easy for the user to insert the tray 101 into the main body and to pull the tray 101 out of the main body.
Moreover, the motor 109 must be located at a position where the end 106a of the shaft 106 engages the coupling 110 of the motor 109 when the tray 101 is inserted in the main body. As the position of the motor 109 is limited, it becomes difficult to make a compact image forming apparatus.
Further, when the user inserts the tray 101 into the main body, the tip 106a of the shaft 106 may collide with the coupling 110, so that the coupling 110 may be damaged.